Gov. Pat Quinn’s campaign opened a new line of attack Monday, accusing Republican challenger Bruce Rauner of channeling personal investments through the Cayman Islands to avoid U.S. taxes.

“The Cayman Islands is like out of central casting when we talk about notorious tax shelters, not only for avoiding taxes but also for avoiding disclosure,” Quinn running mate Paul Vallas said at a news conference.

Such comments are aimed at leveraging the political stigma that clings to offshore investments, as Republican presidential contender Mitt Romney — a wealthy equity investor like Rauner — found out in 2012 when Democrats blistered him for holdings in several so-called foreign tax havens.

That said, experts on tax avoidance techniques interviewed by the Tribune say investments in places like the Caymans benefit some classes of investors but typically not individuals like Rauner.For his part, Rauner has dismissed as “political spin” a published report about several Cayman holdings in his portfolio.

Republican governor candidate Bruce Rauner urged Asian-Americans to vote for him over Democratic Gov. Pat Quinn during a stop in Chinatown today, with a former member of a rival GOP ticket encouraging voters to look beyond party labels. Rauner, the GOP nominee, received encouragement from Steve...

Republican governor candidate Bruce Rauner urged Asian-Americans to vote for him over Democratic Gov. Pat Quinn during a stop in Chinatown today, with a former member of a rival GOP ticket encouraging voters to look beyond party labels. Rauner, the GOP nominee, received encouragement from Steve... (Rick Pearson)

The Cayman issue is the latest in a series of tax-related attacks by Quinn since the Tribune reported last month that Rauner, one of the nation’s richest tax filers, paid taxes on most of his millions at 15 percent, a rate less than half the top federal rate for the wealthy. To accomplish that, Rauner relied on tax reducing strategies out of reach for most taxpayers, including one now facing IRS scrutiny that centered on investment fees paid to his private equity firm, the Tribune reported.

Gov. Pat Quinn today formally received the backing of the Illinois Federation of Teachers, which joined the Illinois Education Association in endorsing the Democratic governor over Republican challenger Bruce Rauner.The IFT’s endorsement was not a surprise, given it is part of a public employee...

Gov. Pat Quinn today formally received the backing of the Illinois Federation of Teachers, which joined the Illinois Education Association in endorsing the Democratic governor over Republican challenger Bruce Rauner.The IFT’s endorsement was not a surprise, given it is part of a public employee... (Rick Pearson)

Rauner’s offshore investments are an offshoot of his long and lucrative ties to GTCR, the Chicago private equity firm he headed for years before leaving in 2012 to ready his run for governor. In March, GTCR submitted documents to the Securities and Exchange Commission declaring its ownership of six investments pools registered in the Caymans and valued collectively in the hundreds of millions of dollars.

An economic disclosure filed with the state by Rauner in conjunction with his candidacy lists him as an investor in several of GTCR’s Cayman based funds. Those records, however, do not reveal the value of Rauner’s stake.

Separately, the SEC filing showed that Rauner maintains close financial ties to GTCR despite his departure. It lists Rauner as a partner in GTCR Management X LP, a subsidiary tied to one of its investment pools with $3.8 billion in assets. The document also listed Rauner as owning a stake in that subsidiary worth between 10 percent and 25 percent.

During a June interview, the Tribune asked Rauner about the SEC documents that listed GTCR’s Cayman investments. “I don’t think that’s true,” Rauner said at the time. “No GTCR fund that I'm aware of has its base in the Cayman Islands.”

On Sunday, however, Rauner offered a different answer when asked about the firm’s Cayman investments: “GTCR has its own structure for just a couple of investments. When they invest in overseas companies, they set up that particular structure.”

In the hothouse atmosphere of a political campaign, Cayman-based investments provide an inviting target. The tiny Caribbean islands south of Cuba are home to just 55,000 people but 93,000 registered companies, many positioned there to take advantage of loopholes that shield foreigners and U.S. based pension funds and endowments from special U.S. taxes on profits on investments parked in tax havens.

The GTCR Cayman investments appear to be in financial instruments typically referred to as blocker funds or alternative investment vehicles that are legally walled off for tax purposes from related investments based in the United States. The arrangement helps non-profit institutional investors avoid taxes.

But experts say it typically provides no tax advantage for individual American investors like Rauner, who are required by U.S. law to pay taxes on all income earned worldwide.

“It’s a common structure that in my view is not abusive in the way that people might think when they see the name Cayman Islands,” explained Victor Fleischer, a professor who teaches tax law at the University of San Diego and has written extensively about tax avoidance strategies.

Even so, the New York Times reported in 2012 that Romney, who publicly released two years worth of complete tax returns during his presidential run, may have used offshore investments to avoid some special taxes in a different way. The newspaper said it involved debt financing that could have been levied against Romney’s normally tax exempt Individual Retirement Account.

It is impossible to conduct such an analysis involving Rauner, who has released three years of basic tax filings but declined to make public supporting schedules and forms that provide significant detail about his financial picture.

Illinois law does not require any tax disclosure by officeholders or political candidates, but many — Quinn included — routinely release their complete tax returns in a show of transparency.

Rauner’s reluctance to follow suit has created a political opening for Quinn, whose campaign is seeking to portray the Republican as a pampered elitist.

“He’s saying ‘trust me, trust me that I did everything legal.’ Well, trust is a two-way street,” Vallas said. “We’re not competing to become CEO of a corporation, we’re competing for the governorship of the state of Illinois… this is a public office that requires public disclosure.”